1. Field of the Invention
Aspects of the present invention relate to an optical pickup device, and more particularly, to an optical pickup device, which reduces a size of an optical spot formed on a multilayer optical disk having a plurality of recording layers and prevents degradation of a tracking error signal due to a layer adjacent a target recording layer of the optical disk while the target recording layer is reproduced.
2. Description of the Related Art
An optical disk is a storage medium recording/reproducing data, such as sound, image, and documents, by changing reflection of laser light by pits formed on a surface of the optical disk. Types of optical disks include a Compact Disk (CD) and a Digital Versatile Disk (DVD). Recently, research in high-density recording media having an increased recording capacity as a new-generation optical disk has been vigorously carried out. These high-density optical disks include a Blu-ray Disk (BD) and an Advanced Optical Disk (AOD). Optical disks are recorded and reproduced by an optical recording/reproducing device using a laser light source having a varied wavelength and an object lens having a varied numerical aperture according to the amount of stored data.
For example, in the BD, which has a capacity higher than a capacity of the DVD, the wavelength of the light source is 405 nm, and the numerical aperture of the object lens is 0.65. In order to prevent degradation of performance of the BD due to an inclination of the BD, a thickness of the BD is approximately 0.1 mm.
Further, in order to increase the capacity of the optical disk, the BD employs a multilayer structure having a plurality of recording layers.
The recording capacity of the optical recording/reproducing device, which records data on an optical disk, serving as an optical data storage medium, using an optical spot obtained by concentrating laser light by an object lens, or reproduces the recorded data, is determined by the size of the optical spot. The size (S) of the optical spot may be expressed by equation (1):S∝k*λ/NA,  (1)where: λ is a wavelength of the laser light, NA is a numerical aperture of the object lens, and k is a constant in a range of 1 to 2, depending on an optical system.
Accordingly, in order to increase the density of an optical disk, the size (S) of an optical spot formed on the optical disk must be reduced. In order to reduce the size (S) of the optical spot, the wavelength (λ) of laser light is reduced or the numerical aperture (NA) of the object lens is increased, as described by the above equation.
In order to reduce the wavelength (λ) of the laser light, high-priced components are required. Further, where the numerical aperture (NA) of the object lens is increased, a depth of focus of the object lens is decreased in proportion to a square of the numerical aperture (NA), i.e., NA2, and comma aberration of the object lens is increased in proportion to a cube of the numerical aperture (NA), i.e., NA3. Accordingly, decreasing the wavelength and increasing the numerical aperture each has a limit in reducing the size (S) of the optical spot.
Where a multilayer optical disk having a plurality of recording layers, for example, a double layer optical disk having two recording layers, uses a Differential Push-Pull (DPP) method for detecting a tracking error signal, zero-order light reflected by an adjacent reproducing layer of a target reproducing layer overlaps with positive and negative first order-light reflected by the target reproducing layer, thereby causing degradation of a tracking error signal.
In order to address the above problem, Korean Patent Laid-open Publication No. 10-2005-0074839 discloses a separate optical member, particularly a polarizing hologram, which diffracts zero-order light reflected by an adjacent layer of a target layer so that the diffracted zero-order light is not received by first and second sub optical detectors. When the center of the polarizing hologram does not coincide with the optical axis of an object lens, the zero-order light reflected by the adjacent layer is partially incident on the first and second sub optical detectors, thereby exerting a negative influence on the quality of a tracking error signal.